1. Field of the Invention
The present invention relates to a method for manufacturing an optical element comprising an optical element in the form of a polymer flat sheet (hereinafter referred to as polymer film), which is used as an optical component such as an optical branch coupler, optical transceiver module, spatial light modulator, and microlens array, and more particularly to an optical element which can be easily mass produced at a low cost and with high dimensional precision as an optical waveguide having a distributed index in a polymer in the form of a flat sheet, and as an optical element involving the use of a series of polymers having an optical phase controlling or optical path controlling function.
2. Description of the Related Art
Optical elements and optical components, such as optical branch couplers, optical transceiver modules, spatial light modulators, and microlens arrays, have rapidly come to enjoy wide use in a wide range of fields involving the use of light, such as the data communications, data processing, and imaging fields. Their increasing use in these fields has led to greater demand for higher performance and lower costs.
Optical elements involving the use of polymer film have excellent properties, such as their simple manufacture, inexpensive starting materials, broad application in the form of films, and inexpensive manufacturing equipment. Some drawbacks, however, are that shrinkage caused by solvent extraction during the manufacture of polymer films results in poor dimensional precision, with substantial differences in the thermal expansion coefficient between inorganic compounds.
A method referred to as selective photopolymerization has been disclosed in Japanese Patent Publication 56-3522 as a method for manufacturing optical elements using polymer film. This is a method in which a distributed index is formed inside the film. A specific manufacturing method has been disclosed in Japanese Patent Laid-Open Publication 3-156407.
In this method, a polycarbonate resin obtained using bisphenol Z as the starting material is used as a transparent polymer, and a vinyl monomer such as methyl acrylate is used as a photopolymerizable monomer, so as to manufacture a film consisting of a polycarbonate Z containing a photopolymerizable monomer. Desired portions of the film are then exposed to polymerize and fix these portions, and the unreacted monomer is dried away. The aforementioned step results in portions with differing refractivities (the exposed portions contain acrylic resin, resulting in a lower refractivity).
The specific manufacturing steps for obtaining an optical element using a polymer film comprise the following series of steps.
Step 1: A transparent polymer film containing a photopolymerizable monomer is manufactured by solvent casting in a container.
Step 2: Desired portions of the polymer film obtained in Step 1 are irradiated with ultraviolet rays to polymerize and fix the photopolymerizable monomer in the exposed portions.
Step 3: The exposed polymer film is separated and dried, and the unreacted photopolymerizable monomer is removed to form a distributed index.
Step 4: The dried polymer film is allowed to adhere to a glass substrate.
Step 5: Subsequent processing results in the desired optical element.
The steps are described in greater detail below.
The solvent casting method is employed in Step 1 because the film is simple to manufacture, the film thickness can be readily controlled, and the concentration of the photopolymerizable monomer is readily controlled, allowing desired differences in refractivity to be obtained.
The film is manufactured in this method by allowing a polymer solution to flow into a flat-bottomed container, and evaporating the solvent off as the solvent vapor pressure is controlled. There is a minimum concentration for the matrix resin and the photopolymerizable monomer used to obtain a distributed index by photopolymerization in the polymer solution used here. The solution may also contain a sensitizer in the form of a photopolymerization promoter as needed. A highly transparent polycarbonate resin and an acrylic-based monomer are frequently used as the matrix polymer and photopolymerizable monomer material, respectively.
The film thickness is controlled by the ratio between the container floor surface area and the amount of solution, and by the concentration of the solution.
The film thickness distribution is also adjusted by the horizontality and flatness of the container and the solvent drying conditions.
A polymer film containing a suitable amount of photopolymerizable monomer is obtained by these operations. After the solvent has been sufficiently evaporated off, the polymer film can be readily separated from the container whenever necessary.
In Step 2, the film that has been obtained in Step 1 is exposed to ultraviolet rays after a photomask having a light-blocking pattern has been placed on the film. This allows the photopolymerizable monomer in the film to be polymerized by the ultraviolet light rays and thus fixed in the film.
In Step 3, the polymer film is then separated from the container, the film is dried, and the unreacted photopolymerizable monomer in the matrix is removed. Here, in conventional cases, the film is removed during drying when the polymer film is dried without being removed from the container.
As a result of this drying, the film dimensions shrink about 2 to 4% because of the evaporation of the unreacted photopolymerizable monomer or residual solvent in the film. If the film is of a uniform thickness, the shrinkage is entirely uniform. The shrinkage is not uniform, however, if the film thickness is not uniform, resulting in warpage. Although the rate of shrinkage can be predicted to a certain extent, it is difficult to prevent dimensional error of about 10 .mu.m per centimeter.
In Step 4, the distributed index polymer film manufactured in the above steps is allowed to adhere on a resin sheet or glass plate serving as the base.
In Step 5, the target optical element is manufactured by subsequent processes in which the film is cut to the necessary size and is combined as needed with other materials or the like.
The problem of dimensional precision due to the aforementioned shrinkage is a major problem in the manufacturing process.
The precision of recent optical products has improved considerably, and it is now easier to align optical axes, which used to be a factor in the greater costs in the past because of the troubles involved. In image-managing fields involving spatial light modulators and microlens arrays, increases in surface area have led to demand for location precision with display photoreception elements.
Conventional methods for manufacturing polymer films were simple, and a certain degree of precision was easy to obtain, depending on the solvent concentration, drying method, and the like. However, their precision is inadequate compared to the precision now demanded of the recent optical components described above. That is because of the poor dimensional precision and poor reproducibility of the pattern that is obtained owing to the shrinkage that occurs when the film is dried.
Although the rate of shrinkage can be predicted and controlled to a certain extent by controlling the drying conditions and the concentration of the photopolymerizable monomer, it is difficult to control the dimensions to within 10 .mu.m per centimeter.
Although it is necessary to adjust the polymer and photopolymerizable monomer concentrations to address the extent of the variation in refractivity and thickness of the film that is manufactured, the need for manufacturing separate masks because of changes in the rate of shrinkage in such cases is an inconvenience.
When the film is no more than 20 .mu.m, allowing the film to adhere to the base after drying makes handling difficult, it is difficult to have it adhere with good precision, and it is difficult to ensure the location precision of the element that is obtained.
For these reasons, it is no easy feat to form a pattern with high precision on a polymer film, and to obtain an optical element while retaining that precision.
In addition, because of the substantial thermal expansion coefficient of polymer films, the usable temperature range is sometimes limited in many applications requiring dimensional precision.
Optical elements obtained using polymer films have the advantages of inexpensive material costs and ease of manufacturing. However, because of the difficulties in ensuring precision, it has been difficult to lower the costs of devices as a result of the trouble involved in alignment, poor yields, and the like.